1. Field of the Invention
The present invention relates to a solid state image pickup apparatus, and more particularly to a solid state image pickup apparatus having a floating diffusion unit adjacent to the photoelectric conversion unit.
2. Related Background Art
Conventionally, the charge coupled device (CCD) is widely utilized as the solid state image pickup apparatus. FIG. 1 is a schematic cross-sectional view of a common CCD, and FIG. 2 shows the change of the potential distribution in time of various areas shown in FIG. 1, wherein areas same as those in FIG. 1 are represented by same numbers.
In FIG. 1 there are shown a Si semiconductor substrate 1 of a first conductive type a well area 2 of a second conductive type opposite to that of the substrate 1, and a diffusion area 3 of the first conductive type, constituting a PN photodiode with the well area 2. The PN photodiode is initially given an inverse bias and a photo-induced charge is accumulated according to the incident light amount, in the junction capacitance of the PN photodiode. There are also shown a shallow diffusion area 4 of the second conductive type, provided for reducing the dark current generated on the surface of the diffusion area 3 and forming a diode also between the areas 3 and 4 thereby increasing the amount of the accumulated charge, a transfer channel 5 for reading the photo-induced charge accumulated in the photodiode, and a CCD channel 6 for successive transfer of the read charge.
A control electrode 7 serves to control the charge transfer to the CCD and also the successive charge transfer in the CCD, and is formed on the transfer channel 5 and the CCD channel 6 across an insulation layer. Second, third and fourth electrodes 8, 9, 10 serve to transfer the charge in the CCD in succession. A floating diffusion area 11 finally receives the transferred photo-induced charge, and a voltage amplitude, generated in the floating diffusion area 11 according to the photo-induced charge amount, is detected by a source follower amplifier, omitted in FIG. 1, and outputted to the outside as an electrical signal. There are also shown a reset gate 12 for resetting the floating diffusion area 11, and a reset drain 13 therefor.
In the following the functions will be explained with reference to FIG. 2, which illustrates the potential states of the various areas. At a time t0, the transfer channel 5 is turned off and the photodiode area 2, 3 accumulates the photo-induced charge corresponding to the incident light amount. At a time t1, the transfer channel 5 is turned on to transfer the photo-induced charge to the CCD channel 6 through the transfer channel 5.
At times t2, t3, the control electrodes 8, 9 are controlled in succession to transfer the photo-induced charge is transferred in succession to the portions in the CCD channel 6, positioned directly below the control electrodes. The floating diffusion area 11 is reset in advance at the time t3, and, the photo-induced charge is transferred by the control electrode 10, at a time t4, to the floating diffusion area 11 and is converted therein into a voltage for output.
In the above-described configuration, however, as the common control electrode 7 is used for controlling the potentials of the transfer channel 5 and the first CCD channel 6, the difference in the depth of the potential wells thereof is uniquely determined by the process conditions such as the concentration profile. For this reason, there may result a case where the depth of the potential well of the CCD channel 6 cannot be sufficiently secured as shown in FIG. 3 in which areas same as those in FIG. 1 are represented by same numbers. As a result, the photo-induced charge cannot be fully transferred but partly remains in the transfer channel 5, leading to defective phenomena such as:
(1) a decrease in the total amount of the transferred charge, leading to a lowered sensitivity; and
(2) return of the charge remaining in the transfer channel 5 to the photodiode 2, 3, thereby generating a retentive image and significantly deteriorating the image quality.
Also, Japanese Patent Application Laid-Open No. 2-30189 discloses a configuration having an electrode for controlling the charge transfer from the photodiode to the CCD channel and another electrode for controlling the charge transfer in the CCD, as shown in FIG. 5. Referring to FIG. 5, a plurality of n-type accumulation areas 1 for accumulating the signal charges induced by the incident light are arranged on a p-type substrate 6, and a transfer gate 3 is provided between the accumulation area 1 and a vertical CCD register 2, of which an end is connected to a horizontal CCD register composed of a transfer gate 12, a transfer electrode 13 for the vertical CCD register and a silicon dioxide layer 14. VA denotes the potential of the n-type accumulation area 1, taking the Fermi potential 15 of the interior of the p-type substrate as a reference, while VDEP denotes the potential VA required for completely depleting the accumulation area 1, and Vch denotes the channel potential when the transfer gate 3 is turned on. The potential VA is lowered in the drawing in response to the light entering the accumulation area 1, and the accumulated charge is transferred to the vertical CCD register 2 by lowering the potential of the gate 12.
However, according to the above-mentioned patent application, the depth of the potential well of the CCD channel is still uniquely determined by the process conditions such as the concentration profile, so that there are similarly encountered the above-mentioned drawbacks.
In the field of solid state image pickup apparatus, in addition to the CCD, the MOS sensor is recently attracting attention and is being actively developed because of advantages such as ease of one-chip formation of the peripheral circuits. For example, an exhibit titled “An Active Pixel Sensor Fabricated Using CMOS/CCD Process Technology” (exhibited at the 95 IEEE WORKSHOP on Charge-Coupled Devices and Advanced Image Sensors) discloses a CMOS sensor and its potential chart as shown in FIG. 4. As shown in FIG. 4, in the CMOS sensor, the floating diffusion area 711 and the source follower amplifier (not shown) are provided for each pixel, instead of at the end of the CCD register in case of the CCD. In FIG. 4, components same as those in FIG. 1 are represented by same numbers. There are also shown a transfer gate 701 provided for each pixel, and a floating diffusion area 711 provided for each pixel. The photo-induced charge, generated in the photodiode area is transferred to the floating diffusion area 711, and the amplitude of the voltage generated therein according to amount of photo-induced charges is detected by the source follower amplifier, omitted in FIG. 4 but provided for each pixel, and outputted to an output line through a pixel selecting switch. There are also shown a reset gate 712 for resetting the floating diffusion area 711 provided for each pixel, and a reset drain 713 therefor.
Such circuit configuration of transferring the charge of each pixel to the corresponding floating diffusion area 711 and effecting the selection of pixels by a common MOS circuit allows to achieve both a high S/N ratio as the sensor and a high performance realized by a one-chip MOS circuit.
However such conventional configuration is still associated with the drawbacks similar to those in the conventional CCD configuration, because of changes in the reset voltage in the floating diffusion area and in the depth of the potential well of the transfer channel, caused by a fluctuation in the manufacturing process conditions. These drawbacks have not been paid any attention, and the potential chart of the aforementioned exhibit only showed a configuration that is incapable of complete transfer of the charge from the photodiode to the floating diffusion area.